In our latest mining experts series we interviewed Daniel Rioux. We talked about density control in the thickening process. Daniel is an instrumentation technologist at a nickel mining operation in Sudbury, Ontario, Canada.
Q: What are the first steps for implementation of density measurement and control?
A: First of all, the installation step is the most important one, as mistakes are made with selecting the pipe sizing, the location, and the orientation of the meter. These are all key components for a proper installation. It’s important to install it correctly right away, because it’s very difficult to correct after the fact and it can be costly to rework.
Q: So, if the instrument is not installed in the right way, the measured results may deviate from the real density value?
A: Yes, it definitely can. When we are talking about slurries, settling is always kind of a challenge we have. So settling will tend to build up in front of or within the measurement zone and stack into the measurement result and caused deviations. That settling is usually very inconsistent. You want to measure the density regardless of any kind of settling effects. So you choose the best location for it.
Q: The orientation of the instrument in regard to the pipe is important, but what about the pipe itself, does the orientation of the pipe also matter?
A: Yes. Vertical installations are usually best, because settling in a vertical pipe is not really possible.
Q: Ok, so the instrument is installed, now what else do we need to take into account?
A: You need to check for dead time to be able to set up effective control actions.
Q: What is dead time?
A: Dead time is the point of where you made the change to the point where you actually see the change. It is the physical time between the point of the control action, and the point where the result is measured. That is a problem because the standard control expects a response right away. Think of a long conveyor, fed at one end, yet a measurement occurs further down. Dead time is a big problem for controllers, as it requires very heavily reduced control responses. This becomes problematic during start-up conditions, during any disturbance like varying dilution water pressure, or during setpoint changes.
Q: What if there is a dead time in the vertical section that was chosen as installation point?
A: What happens is the thickener pulls from the centre. Thickeners are typically quite broad in diameter. By the time you get to the vertical section of piping, you end up with a big delay at dead time. From the point where you actually implementing density control, meaning you are making a change, and then sensing it in the meter, there is going to be a big delay between those two. Assuming there will be some density control applied (like trim dilution), the distance from the dilution point at the centre of the thickener footprint, to the vertical section at the perimeter can result in considerable dead time.
In the next blog you can read how to solve the dead time and which control actions you can use.